Reduction of polymerization inhibitor irregularity on additive manufacturing windows

ABSTRACT

An apparatus for producing a three-dimensional object by additive manufacturing includes a drive assembly operatively associated with a carrier and a window mount and configured to advance the carrier and the window mount away from one another. A first fluid switch is connected to a first fluid orifice when a window is present, or connected to the window mount and configured for connection to the window when the window is absent. A second fluid switch is connected to a second fluid orifice when the window is present, or connected to the window mount and configured for connection to the window when the window is absent. A fluid supply is connected to both the first and second fluid switch, and the fluid includes a polymerization inhibitor.

RELATED APPLICATIONS

This application is divisional of U.S. patent application Ser. No.16/490,137, filed Aug. 30, 2019, which application is a 35 U.S.C. § 371national phase entry of International Application No. PCT/US2018/057273,filed Oct. 24, 2018, which claims priority U.S. Provisional ApplicationSer. No. 62/578,008, filed Oct. 27, 2017, the disclosures of which areincorporated by reference herein in their entireties.

FIELD OF THE INVENTION

The present invention concerns additive manufacturing, particularlystereolithography, including continuous liquid interface production(CLIP).

BACKGROUND OF THE INVENTION

A group of additive manufacturing techniques sometimes referred to as“stereolithography” create a three-dimensional object by the sequentialpolymerization of a light polymerizable resin. Such techniques may be“bottom-up” techniques, where light is projected into the resin on thebottom of the growing object through a light transmissive window, or“top down” techniques, where light is projected onto the resin on top ofthe growing object, which is then immersed downward into the pool ofresin.

The recent introduction of a more rapid stereolithography techniqueknown as continuous liquid interface production (CLIP), coupled with theintroduction of “dual cure” resins for additive manufacturing, hasexpanded the usefulness of stereolithography from prototyping tomanufacturing (see, e.g., U.S. Pat. Nos. 9,211,678; 9,205,601; and9,216,546 to DeSimone et al.; and also in J. Tumbleston, D.Shirvanyants, N. Ermoshkin et al., Continuous liquid interfaceproduction of 3D Objects, Science 347, 1349-1352 (published online 16Mar. 2015); see also Rolland et al., U.S. Pat. Nos. 9,676,963, 9,453,142and 9,598,606).

Techniques such as CLIP harness inhibitors of polymerization such asoxygen to the benefit of the stereolithography process. However, theinhibitors can be consumed, and when consumed must be re-supplied,particularly when the process is operated rapidly. And rapid operationof the process can lead to seemingly unrelated additional problems, suchas cavitation and bubble formation in the polymerizable liquid beneaththe growing three-dimensional object, leading to defect formation withinthe object itself. Accordingly, a need remains for new techniques inbottom-up stereolithography.

SUMMARY

In some embodiments, an apparatus for producing a three-dimensionalobject by additive manufacturing includes (a) a light source; (b) acarrier or carrier mount positioned above the light source, on whichcarrier a three-dimensional object can be produced; (c) a window mount;(d) optionally, a fixed or removable window cassette connected to thewindow mount; the window comprising: (i) an optically transparentsemipermeable member, on which member a three-dimensional object can beproduced; (ii) a fluid supply bed in or adjacent the semipermeablemember and configured to feed a polymerization inhibitor through thesemipermeable member, the supply bed having at least two opposing sides;(iii) at least a first and second fluid orifice connected to the supplybed on opposite sides thereof and in fluid communication with oneanother through the supply bed; (e) a drive assembly operativelyassociated with the carrier and the window mount (and the windowcassette when present) and configured to advance the carrier and thewindow mount (and the window cassette when present) away from oneanother; (f) a first fluid switch connected to the first fluid orificewhen the window is present, or connected to the window mount andconfigured for connection to the window when the window is absent; (g) asecond fluid switch connected to the second fluid orifice when thewindow is present, or connected to the window mount and configured forconnection to the window when the window is absent; (f) a fluid supplyconnected to both the first and second fluid switch, the fluidcomprising a polymerization inhibitor (e.g., oxygen); and (g)optionally, a vacuum source connected to both the first and second fluidswitch.

In some embodiments, the first and second fluid switches are togetherconfigured to be switchable between: (i) a first configuration in which:the fluid inhibitor supply is in fluid communication with the firstfluid orifice; and the vacuum source, if present, is in fluidcommunication with the second fluid orifice; and (ii) a secondconfiguration in which: the fluid inhibitor supply is in fluidcommunication with the second fluid orifice; and the vacuum source, ifpresent, is in fluid communication with the first fluid orifice.

In some embodiments, the fluid supply bed comprises a lateral flow bed(e.g., positioned between a semipermeable member and an impermeablemember) (e.g., a fluid channel bed).

In some embodiments, the vacuum source is present.

In some embodiments, the fluid comprises a gas.

In some embodiments, the polymerization inhibitor comprises oxygen.

In some embodiments, the fluid comprises an oxygen-enriched gas at apressure less than atmospheric pressure.

In some embodiments, the window cassette is present.

In some embodiments, the window cassette is absent.

In some embodiments, the semipermeable member comprises an amorphousfluoropolymer.

In some embodiments, a method of reducing irregularity of an oxygengradient across the window of a bottom-up additive manufacturingapparatus during production of a three-dimensional object thereonincludes (a) producing the object from a polymerizable liquid on awindow, the window comprising: (i) an optically transparentsemipermeable member, on which member the three-dimensional object isproduced; (ii) a fluid supply bed in or adjacent the semipermeablemember, from which supply bed a polymerization inhibitor is fed throughthe semipermeable member; (iii) at least a first and second fluidorifice connected to the supply bed on opposite sides thereof and influid communication with one another through the supply bed; while (b)feeding a fluid comprising a polymerization inhibitor (e.g., oxygen)through one of the orifices, optionally while drawing a vacuum throughthe other of the orifices; and (c) periodically reversing the flow ofthe fluid through the supply bed.

In some embodiments, the fluid supply bed comprises a lateral flow bed(e.g., positioned between a semipermeable member and an impermeablemember) (e.g., a fluid channel bed).

In some embodiments, the step of drawing a vacuum is present.

In some embodiments, the periodically reversing step is carried out by(i) periodically switching the orifice through which the inhibitor ofpolymerization is fed, and (ii) the orifice from which the vacuum, ifpresent, is drawn.

In some embodiments, the fluid comprises a gas.

In some embodiments, the fluid comprises an oxygen-enriched gas at apressure less than atmospheric pressure.

In some embodiments, the pressure of the oxygen-enriched gas issubstantially equal to a partial pressure of oxygen in air atatmospheric pressure.

In some embodiments, the step of drawing a vacuum is appliedintermittently.

In some embodiments, the polymerizable liquid comprises a lower regionon the optically transparent member, and an upper region on the lowerregion opposite the optically transparent member, wherein the step ofapplying a reduced pressure and/or polymer inhibitor-enriched gas to thepolymerizable liquid through the optically transparent member comprisesreducing a gas (e.g., nitrogen) concentration in the upper region.

In some embodiments, a thickness of the lower region is about 1 to 1000microns.

In some embodiments, the step of intermittently applying a reducedpressure to the polymerizable liquid through the optically transparentmember is sufficient to reduce a gas (e.g., nitrogen) concentration inthe upper region and maintain a gas (e.g., oxygen) concentration in thelower region.

In some embodiments, the step of periodically switching is carried outat a frequency of from 0.01, 0.02, or 0.1 Hertz, up to 0.2, 0.5, 1, or 2Hertz.

The foregoing and other objects and aspects of the present invention areexplained in greater detail in the drawings herein and the specificationset forth below. The disclosures of all United States patent referencescited herein are to be incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates, for comparative purposes, a bottom-upstereolithography apparatus in which a fluid comprising a polymerizationinhibitor is flowed laterally beneath a semipermeable window.

FIG. 2 is a top down view of a window of an apparatus of FIG. 1 , withdarker regions showing regions of polymerizable liquid in which oxygenis depleted, and hence susceptible to collapse of the sustained liquidinterface release layer (or “dead zone”).

FIG. 3 is a side-sectional view of a window for a bottom-upstereolithography apparatus in which oxygen is passed through the windowinto the polymerizable liquid, showing that, as oxygen concentration inthe polymerizable liquid is increased, nitrogen concentration(responsible for undesirable bubble formation in objects being produced)is also advantageously reduced.

FIG. 4 is a schematic illustration of an apparatus of the presentinvention.

FIG. 5 is similar to FIG. 2 , except taken from an apparatus of FIG. 4 .Note the absence of highly oxygen-depleted regions.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention is now described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein; rather these embodiments are provided sothat this disclosure will be thorough and complete and will fully conveythe scope of the invention to those skilled in the art.

Like numbers refer to like elements throughout. In the figures, thethickness of certain lines, layers, components, elements or features maybe exaggerated for clarity. Where used, broken lines illustrate optionalfeatures or operations unless specified otherwise.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises” or“comprising,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements components and/orgroups or combinations thereof, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components and/or groups or combinations thereof.

As used herein, the term “and/or” includes any and all possiblecombinations or one or more of the associated listed items, as well asthe lack of combinations when interpreted in the alternative (“or”).

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the specification andclaims and should not be interpreted in an idealized or overly formalsense unless expressly so defined herein. Well-known functions orconstructions may not be described in detail for brevity and/or clarity.

It will be understood that when an element is referred to as being “on,”“attached” to, “connected” to, “coupled” with, “contacting,” etc.,another element, it can be directly on, attached to, connected to,coupled with and/or contacting the other element or intervening elementscan also be present. In contrast, when an element is referred to asbeing, for example, “directly on,” “directly attached” to, “directlyconnected” to, “directly coupled” with or “directly contacting” anotherelement, there are no intervening elements present. It will also beappreciated by those of skill in the art that references to a structureor feature that is disposed “adjacent” another feature can have portionsthat overlap or underlie the adjacent feature.

Spatially relative terms, such as “under,” “below,” “lower,” “over,”“upper” and the like, may be used herein for ease of description todescribe an element's or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is inverted, elements described as “under” or “beneath” otherelements or features would then be oriented “over” the other elements orfeatures. Thus the exemplary term “under” can encompass both anorientation of over and under. The device may otherwise be oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly. Similarly, the terms“upwardly,” “downwardly,” “vertical,” “horizontal” and the like are usedherein for the purpose of explanation only, unless specificallyindicated otherwise.

It will be understood that, although the terms first, second, etc., maybe used herein to describe various elements, components, regions, layersand/or sections, these elements, components, regions, layers and/orsections should not be limited by these terms. Rather, these terms areonly used to distinguish one element, component, region, layer and/orsection, from another element, component, region, layer and/or section.Thus, a first element, component, region, layer or section discussedherein could be termed a second element, component, region, layer orsection without departing from the teachings of the present invention.The sequence of operations (or steps) is not limited to the orderpresented in the claims or figures unless specifically indicatedotherwise.

1. Additive Manufacturing Methods and Apparatus.

Additive manufacturing apparatus and methods are known. Suitableapparatus includes bottom-up apparatus that employ a window, oroptically transparent member or “build plate,” on which a pool ofpolymerizable liquid sits, and through which patterned light isprojected to produce a three-dimensional object. Such methods andapparatus are known and described in, for example, U.S. Pat. No.5,236,637 to Hull, U.S. Pat. Nos. 5,391,072 and 5,529,473 to Lawton,U.S. Pat. No. 7,438,846 to John, U.S. Pat. No. 7,892,474 to Shkolnik,U.S. Pat. No. 8,110,135 to El-Siblani, U.S. Patent ApplicationPublication Nos. 2013/0292862 to Joyce, and US Patent ApplicationPublication No. 2013/0295212 to Chen et al. The disclosures of thesepatents and applications are incorporated by reference herein in theirentirety.

CLIP is known and described in, for example, U.S. Pat. Nos. 9,211,678;9,205,601; and 9,216,546 to DeSimone et al.; and also in J. Tumbleston,D. Shirvanyants, N. Ermoshkin et al., Continuous liquid interfaceproduction of 3D Objects, Science 347, 1349-1352 (published online 16Mar. 2015). See also R. Janusziewcz et al., Layerless fabrication withcontinuous liquid interface production, Proc. Natl. Acad. Sci. USA 113,11703-11708 (Oct. 18, 2016). In some embodiments, CLIP employs featuresof a bottom-up three dimensional fabrication as described above, but thethe irradiating and/or the advancing steps are carried out while alsoconcurrently maintaining a stable or persistent liquid interface betweenthe growing object and the build surface or window, such as by: (i)continuously maintaining a dead zone of polymerizable liquid in contactwith the build surface, and (ii) continuously maintaining a gradient ofpolymerization zone (such as an active surface) between the dead zoneand the solid polymer and in contact with each thereof, the gradient ofpolymerization zone comprising the first component in partially curedform. In some embodiments of CLIP, the optically transparent membercomprises a semipermeable member (e.g., a fluoropolymer), and thecontinuously maintaining a dead zone is carried out by feeding aninhibitor of polymerization through the optically transparent member,thereby creating a gradient of inhibitor in the dead zone and optionallyin at least a portion of the gradient of polymerization zone. Theparticular manner of description is not critical, and the presentinvention can be used in any of a variety of systems that employ asemipermeable build plate, through which an inhibitor of polymerizationpasses, whether explicitly referred to as “CLIP” or not.

The apparatus can include a local controller that contains and executesoperating instructions for the production of a three dimensional objecton that apparatus, typically from an object data file entered into thecontroller by the user. Along with the basic three-dimensional image ofthe object that is typically projected for photopolymerization (alongwith movement of the carrier and build surface away from one another inthe Z direction), the operating instructions can include or generateprocess parameters such as: light intensity; light exposure duration;inter-exposure duration; speed of production; step height; height and/orduration of upstroke in a stepped or reciprocal operating mode; heightand/or duration of downstroke in a reciprocal operating mode; rotationspeed for pumping viscous polymerizable liquid; resin heatingtemperature; and/or resin cooling temperature; rotation speed andfrequency, etc. (see, e.g., Ermoshkin et al., Three-dimensional printingwith reciprocal feeding of polymerizable liquid PCT Patent ApplicationPub. No. WO 2015/195924 (published 23 Dec. 2015); Sutter et al.,Fabrication of three dimensional objects with multiple operating modes,PCT Patent Application Publication No. WO 2016/140886 (published 9 Sep.2016); J. DeSimone et al., Methods and apparatus for continuous liquidinterface production with rotation, PCT Patent Application WO2016/007495 (published 14 Jan. 2016); see also J. DeSimone et al., U.S.Pat. No. 9,211,678, and J. Batchelder et al., Continuous liquidinterface production system with viscosity pump, US Patent ApplicationPublication No. US 2017/0129169 (published 11 May 2017).

In one non-limiting embodiment, the apparatus may be a Carbon Inc., M1or M2 additive manufacturing apparatus, available from Carbon, Inc.,1089 Mills Way, Redwood City, Calif. 94063 USA.

Numerous resins for use in additive manufacturing are known and can beused in carrying out the present invention. See, e.g., U.S. Pat. No.9,205,601 to DeSimone et al.

In some embodiments, the resin is a dual cure resin. Such resins aredescribed in, for example, Rolland et al., U.S. Pat. Nos. 9,676,963;9,598,606; and 9,453,142, the disclosures of which are incorporatedherein by reference.

Resins may be in any suitable form, including “one pot” resins and “dualprecursor” resins (where cross-reactive constituents are packagedseparately and mixed together before use, and which may be identified asan “A” precursor resin and a “B” precursor resin).

Particular examples of suitable resins include, but are not limited to,Carbon, Inc. rigid polyurethane resin (RPU), flexible polyurethane resin(FPU), elastomeric polyurethane resin (EPU), cyanate ester resin (CE),epoxy resin (EPX), or urethane methacrylate resin (UMA), all availablefrom Carbon, Inc., 1089 Mills Way, Redwood City, Calif. 94063 USA.

2. Build Plates.

In general, a build plate for use in the present invention may compriseany suitable semipermeable or permeable material (that is, permeable tothe polymerization inhibitor) including amorphous fluoropolymers, suchas an amorphous thermoplastic fluoropolymer like TEFLON AF 1600™ orTEFLON AF 2400™ fluoropolymer films, or perfluoropolyether (PFPE),particularly a crosslinked PFPE film, or a crosslinked silicone polymerfilm. Beneath that may be a fluid bed layer, such as provided by a gaspermeable material, optionally containing channels or cavities, such asa permeable polymer (e.g., poly(dimethylsiloxane) (PDMS). A base orsupport member (such as glass or sapphire) may be included at the bottomof the window if necessary, and may serve to further define the fluidsupply bed. The build plate may be supported by a peripheral frame, withthe two together forming a removable window cassette as discussed below.

In some embodiments, the pressure and gas supply to the build plate maybe controlled to reduce bubble or voids formed by excess gasses, such asnitrogen, in the polymerizable fluid (e.g., resin) of in the 3D printingprocess and apparatus. Although the methods described herein may beperformed by controlling a pressure and/or content of the gas suppliedto the build plate using a pressure controller/gas supply, it should beunderstood that any suitable system may be used, including alternativebuild plates. For example, any permeable build plate may be positionedsuch that the side opposite the build surface is in apressure-controlled chamber, or any suitable configuration ofpressure-pressure controlled channels may be used.

The amount and duration of the reduced pressure applied to thepolymerizable liquid through the optically transparent member ispreferably sufficient to reduce a gas concentration in the polymerizableliquid. The pressure may be at 0%, 5%, 10%, 20%, 25%, 30%, 40% to 50%,60%, 70%, 80%, 90% or 100% of atmospheric pressure. The oxygen orpolymerization inhibitor gas composition of the gas supplied may be 20%,25%, 30%, 40% to 50%, 60%, 70%, 80%, 90% or 100% oxygen.

In some embodiments, the polymerizable fluid has a gradient of gasconcentration, which determines an amount of irradiation or “dose” tocure the polymerizable liquid. For example, the polymerizable fluid canhave a lower region on the optically transparent member, and an upperregion on the lower region opposite the optically transparent membersuch that the lower region has a higher dose to cure than the upperregion. The applied reduced pressure to the polymerizable liquid throughthe optically transparent member may reduce a gas concentration in theupper region, while maintaining the polymerization inhibitor gas in thelower region, which consequently reduces a thickness of the dead zone.In some embodiments, the thickness of the lower region is less thanabout 1000 microns or between about 1, 2, 5, 10, 20 50, 100, 200 300 to400, 500, 600, 700, 800, 900 or 1000 microns.

In some embodiments, oxygen gas may be used as the polymerizationinhibitor. Oxygen may be supplied at any suitable pressure, and ispreferably supplied at a pressure that is less than atmosphericpressure. In particular embodiments, the pressure of the oxygen issubstantial equal to a partial pressure of oxygen in air at atmosphericpressure (e.g., 100% oxygen supplied at about 0.2 atm). Thepolymerization inhibitor gas may also be substantially devoid ofnitrogen or other gases that do not substantially contribute topolymerization inhibition in the dead zone.

Without wishing to be bound by any particular theory, resins that aresaturated with gas are prone to degassing when the local pressure drops.Large pressure drops can occur during the build platform movement andresin refill. When the separation of the printed part and window resultin gas coalescence, voids may be formed in the printed part.Accordingly, controlling the pressure of a gas or applying a vacuumthrough the gas permeable build plate may reduce the level of dissolvedgases prior to the pressure change, and reducing an amount of dissolvedgas may increase the pressure differential that the resin can experienceprior to void formation. The build plate is permeable to gasses, andequilibrium may be established at the build plate/resin interfacerelatively quickly. Cycling between air (or oxygen) and vacuum forprinting formation and part movement, respectively, may permit the CLIPprocess to be performed with a maximum pressure differential on theresin prior to void formation the part. Moreover, the removal ofnitrogen, which is not an active component of polymerization inhibition,may reduce the overall gas level and further reduce the formation ofbubbles or voids in the printed part.

In addition, while oxygen delivery to the interface between thepolymerizable fluid and the build plate is desirable, oxygen in theregions of the polymerization fluid that are further away from theinterface may lead to a larger dosage of irradiation to cure thepolymerizable fluid, which results in a longer exposure time and slowerprint speeds. Reducing the overall oxygen level may lead to faster curetimes, by may lead to difficulty maintaining sufficient oxygen at theinterface for the CLIP process to be effective. Moreover, since thelight intensity decays as it passes through the polyermization fluid,the percent monomer to polymer conversions may not be constantthroughout the exposed region. Controlling a level of oxygenconcentration may reduce exposure times and increase print speeds byeffectively maintaining a level of oxygen at the build plate andpolymerization fluid interface. The oxygen concentration profile mayalso be controlled to provide more consistent percent monomer to polymerconversions in view of variations of light intensity.

Additional Build Plate Materials. Any suitable material may be used toform the build plates described herein, including multi-layer buildplates and/or build plates formed of more than one material. Forexample, the flexible layer (used alone or with additional supports orlayers) may include a woven glass fabric (fiberglass or e-glass) with acrosslinked silicone elastomeric coating (such as room temperaturevulcanized (RTV) silicone), which may be lightly infiltrated into theglass fiber fabric to provide mechanical durability. The oxygenpermeability of silicone elastomer (rubber) is similar to Teflon®AF-2400. Such a configuration may be used alone or affixed (adhesivelyadhered) to a glass plate with the unfilled areas of the fabricavailable for air (oxygen) flow. Sulfonated tetrafluoroethylene basedfluoropolymer-copolymers, such as Nafion® from Dupont may also be used.

In some embodiments, asymmetric flat sheet membranes which are currentlyused in very high quantity for water purification applications (see U.S.Patent Publication No. 2014/0290478) may be used. These membranes aregenerally polysulfone or polyethersulfone, and may be coated withperfluoropolymers or crosslinked silicone elastomer to increase chemicalresistance. Also poly(vinylidene fluoride) and possibly polyimideasymmetric (porous) membranes may be used, for example, if chemicalresistance is a problem. Some of the membranes may be used as is withoutcoatings. Examples of such membranes include FilmTec® membranes (DowChemical, Midland, Mich. (USA)). These are porous polysulfone asymmetricmembranes coated with a crosslinked high Tg polyamide (with a coatingthickness of about 0.1 microns). The crosslinked polyamide coatingshould provide chemical resistance. Although the oxygen permeability ofthe polyamide is low, the thickness of the coating may be so low thatthe effective oxygen transmission rate is high. The polysulfone supportwithout the polyamide layer could be coated with a wide variety ofpolymers such as silicone rubber (or AF-2400) to yield very high oxygentransmission. The FilmTec® membranes are produced in very high quantityas they are the prime material used in water desalination plants. PVDFporous membranes may allow repeated use.

Although embodiments according to the present invention are describedwith respect to flexible layers on the build plate that include asemipermeable (or gas permeable) member (e.g., perfluoropolymers, suchas TEFLON AF® fluoropolymers, it should be understood that any suitableflexible material may be used in the configurations described herein.For example, a transparent, resilient paper, such as glassine, may beused. Glassine is a relatively transparent, greaseproof paper formed ofwell-hydrated cellulosic fibers that has been super calendared. Glassinemay be plasticized and/or coated with wax or a glaze. Glassine may begas permeable. In some embodiments, the glassine may be coated with athin layer of crosslinked silicone elastomer or a perfluoropolymer, suchas TEFLON AF® fluoropolymers. Glassine paper is substantially greaseresistant, and may have limited adhesion to the polymerizable liquiddescribed herein.

Build plate coatings. Omniphobic surfaces may be used on the build platesurface or build region. For example, patterned surfaces (either arandom array of particles or micro patterned surfaces) that containnon-miscible fluids that are pinned or held to the surface by capillaryforces may be used. Such a surface may result in fluid on the surfacefloating along the surface. Examples of such surfaces are described inU.S. Pat. Nos. 8,535,779 and 8,574,704, the disclosures of which arehereby incorporated by reference in their entireties.

Examples of build plates that can be used to carry out the presentinvention include, but are not limited to, those described in: U.S. Pat.No. 9,498,920 to J. DeSimone, A. Ermoshkin, and E. Samulski; U.S. Pat.No. 9,360,757 to J. DeSimone, A. Ermoshkin, N. Ermoshkin and E.Samulski; and U.S. Pat. No. 9,205,601 to J. DeSimone, A. Ermoshkin, N.Ermoshkin and E. Samulski; US Patent Application Publication Nos. US2016/0046075 to J. DeSimone, A. Ermoshkin et al.; US 2016/0193786 to D.Moore, A. Ermoshkin et al.; US 2016/0200052 to D. Moore, J. Tumblestonet al.; PCT Patent Application Publication Nos. 2016/123499 to D. Moore,J. Tumbleston et al; WO 2016/123506 to D. Moore, J. Tumbleston et al.;WO 2016/149097 to J. Tumbleston, E. Samulski et al.; WO 2016/149014 toJ. Tumbleston, E. Samulski et al.; and others (the disclosures of all ofwhich are incorporated by reference herein in their entirety).

3. Example Apparatus and Methods.

FIG. 1 shows a bottom-up stereolithography apparatus without flowswitching. The apparatus includes a removable window cassette 11, acarrier platform 12, and a light engine 13. A 3D object 20 is producedon the object from a polymerizable liquid (or “resin”) 19. Drives,controllers, and the like are not shown for clarity but are implementedin accordance with known techniques.

The removable window cassette includes a semipermeable member 11 a, afluid bed layer or region 11 b, and an optional support member 11 c, allsupported by a circumferential frame 11 f. Other aspects and features ofthe window cassette may be as described above.

The apparatus includes an oxygen supply 15 and a vacuum source 16, whichthrough couplings 12 a, 12 b, connect to the window cassette 11, andparticularly to the fluid bed region 11 b, through inlets 11 g withinthe frame 11 f. Inlets may include associated header regions tofacilitate fluid flow and distribution, as is known in the art. Asillustrated, there is an inlet region and an outlet region flowing fromone side of the window to the other A, B.

FIG. 2 illustrates oxygen depletion in a polymerizable liquid resting ontop of a window in an apparatus of FIG. 1 , with progressively darkerregions indicating progressively more oxygen depleted regions. As isapparent, oxygen depletion is highly irregular, and as noted above andfurther illustrated in the side view of FIG. 3 , with oxygen depletion(and the associated risk of “dead zone” failure and consequentproduction failure), comes the risk of excess nitrogen in thepolymerizable liquid leading to bubble formation in the object beingproduced.

FIG. 4 illustrates an apparatus like that of FIG. 1 with similarelements having similar numbers assigned thereto. The internal structureof the window cassette 11 may be the same as that shown in FIG. 1 .However, this apparatus has a pair of flow switch valves 21 a, 21 b,connected to each coupling. Any suitable valve may be used, including3-port solenoid valves, such as an SMC part number NVKF334V-5GS-01Tvalve, available from SMC Pneumatics, 3810 Prospect Ave., Unit A, YorbaLinda, Calif. 92886 USA. By periodically switching the direction of flowof the fluid comprising the polymerization inhibitor, the degree ofoxygen depletion can be made more uniform across the window, asillustrated in FIG. 5 . The frequency of switching will depend onfactors such as the amount of oxygen in the fluid, the internal volumeof the fluid bed in the window, the pressure of the fluid in the bed,the proximity of the flow switches to the window cassette (with closeproximity generally preferred), the speed of object production, and theconfiguration and density of the object being produced, but in generalwill be about 0.01, 0.02, or 0.1 Hertz, up to 0.2, 0.5, 1, or 2 Hertz(that is, flow switches per second), or more, or less.

The foregoing is illustrative of the present invention, and is not to beconstrued as limiting thereof. The invention is defined by the followingclaims, with equivalents of the claims to be included therein.

We claim:
 1. A method of reducing irregularity of an oxygen gradient across the window of a bottom-up additive manufacturing apparatus during production of a three-dimensional object thereon, comprising: (a) producing said object from a polymerizable liquid on a window, said window comprising: (i) an optically transparent semipermeable member, on which semipermeable member said three-dimensional object is produced; (ii) a fluid supply bed in or adjacent said semipermeable member, from which supply bed a polymerization inhibitor is fed through said semipermeable member; (iii) at least a first and second fluid orifice connected to said supply bed on opposite sides thereof and in fluid communication with one another through said supply bed; while (b) feeding a fluid comprising said polymerization inhibitor through one of said orifices, optionally while drawing a vacuum through the other of said orifices; and (c) periodically reversing the flow of the fluid through said supply bed.
 2. The method of claim 1, wherein said fluid supply bed comprises a lateral flow bed.
 3. The method of claim 2, wherein the lateral flow bed is positioned between the semipermeable member and an impermeable member.
 4. The method of claim 1, wherein said step of drawing a vacuum is present.
 5. The method of claim 1, wherein said periodically reversing step is carried out by (i) periodically switching the orifice through which said polymerization inhibitor is fed, and (ii) the orifice from which said vacuum, if present, is drawn.
 6. The method of claim 1, wherein said fluid comprises a gas.
 7. The method of claim 1, wherein said fluid comprises an oxygen-enriched gas at a pressure less than atmospheric pressure.
 8. The method of claim 7, wherein the pressure of the oxygen-enriched gas is substantially equal to a partial pressure of oxygen in air at atmospheric pressure.
 9. The method of claim 1, wherein the step of drawing a vacuum is applied intermittently.
 10. The method of claim 1, wherein the polymerizable liquid comprises a lower region on the optically transparent semipermeable member, and an upper region on the lower region opposite the optically transparent semipermeable member, wherein the step of feeding a fluid comprising said polymerization inhibitor through one of said orifices comprises applying a reduced pressure and/or polymer inhibitor-enriched gas to the polymerizable liquid through the optically transparent semipermeable member and reducing a gas concentration in the upper region.
 11. The method of claim 10, wherein a thickness of the lower region is about 1 to 1000 microns.
 12. The method of claim 10, wherein the step of feeding a fluid comprising said polymerization inhibitor through one of said orifices comprises intermittently applying a reduced pressure to the polymerizable liquid through the optically transparent semipermeable member that is sufficient to reduce a nitrogen gas concentration in the upper region and maintain an oxygen gas concentration in the lower region.
 13. The method of claim 1, wherein said step of periodically switching is carried out at a frequency of from 0.01, 0.02, or 0.1 Hertz, up to 0.2, 0.5, 1, or 2 Hertz.
 14. The method of claim 1, wherein said polymerization inhibitor comprises oxygen. 